Monolithic spectrophotometer

ABSTRACT

A micro spectrophotometer is monolithically constructed on a silicon substrate. The spectrophotometer includes a concave grating, which is used for dispersing optical waves as well as focusing reflected light onto a photodiode array sited on a silicon bridge. The silicon bridge is bent 90° from the surface of the silicon substrate in order to orthogonally intersect the output light from the grating. A precision notch is defined in the silicon substrate for coupling to an optical input fiber. Signal processing circuitry is etched on the substrate using conventional CMOS processes for initial processing of information received from the photodiode array.

Priority is claimed to Provisional patent application, D/98776P, Ser.No. 60/113,240, Filed Dec. 21, 1998, entitled; MONOLITHICSPECTROPHOTOMETER, by Jingkuang Chen and Joel A. Kubby.

FIELD OF THE INVENTION

The present invention is directed to a fully monolithicspectrophotometer on silicon using microelectromechanical structures andCMOS logic circuitry.

BACKGROUND AND SUMMARY OF THE INVENTION

Spectrophotometers are optical instruments which separate opticalsignals according to their wavelengths. They have broad applicationsincluding color identification in flat panel displays or electroniccameras, color control for xerographic printing, optical spectroscopyfor chemical analysis, environmental monitoring, and process controlswhich are related to color identification. Up to date, all commercialspectrophotometers tend to be of rather large size because they areformed by assembling bulky optical elements, mechanical parts,detectors, and microelectronic chips into a system. This currentassembly process needs high precision and is labor intensive, keepingthe cost of conventional bench top spectrophotometers from beingaffordable. There are many additional applications of interest whichwould arise if spectrophotometer were of significantly lower cost,lighter weight, smaller size, rugged, and incorporated signal processingcapability in the instrument. In xerographic printing, aspectrophotometer is a key component in a closed-loop color controlsystem which will enable the printers to generate reproducible colorimages in a networked environment. The development of a compact, lowcost spectrophotometer is thus important ox in realizing highperformance printing systems.

With the advance of micromachining technology, it is now possible tobuild various microstructures, movable mechanical components, microoptical elements, including free-space, out-of-plane lenses andgratings, sensors, and electronic circuits on silicon chips usingmodified IC processes that are able to produce thousands of thesedevices in batch on silicon wafers. Over the past decade, much efforthas been devoted to the development of micro spectrophotometers usingMEMS technology. However, none of these initial efforts were successful,partly because of the technical difficulty associated with theintegration of high-precision optical elements and photodetectors in asystem. The fabrication of these spectrophotometers needs either ahigh-precision wafer-to-wafer bonding or special thin film depositionprocesses for building microgratings or dispersive waveguides on a chip.The alignment of these optical elements with the photodetectors is verycritical. Any misalignment in the scale of as small as one micro meterwill result in significant deviation in device performance. As a result,none of these prototype devices has been commercialized thus far.

This spectrophotometer incorporates concave gratings, photo diode array,and signal processing circuitry on a silicon substrate and issignificantly reduced in size, weight, and cost. On thespectrophotometer chip, the concave gratings for optical wave separationmay be defined using a dry etch on either crystal silicon or polyimide.The optical elements and the photo diode array may be defined usingphotolithography on the same silicon substrate, eliminating thecomplicated alignment and assembling processes which are generallyrequired for fabrication of conventional spectrophotometers. In order toeffectively sense the light reflected from the gratings, the photo diodearray is built on a suspended silicon bridge which is bent 90 degreesfrom the wafer surface. The integration of signal processing circuitryfurther enhances its function and improves the signal-to-noise ratio,resulting in a high resolution spectrum analysis system.

In the present invention, a fully monolithic spectrophotometer onsilicon using MEMS technology is described. This spectrophotometerincorporates concave gratings, a photo diode array, and signalprocessing circuitry on a silicon substrate and is drastically reducedin size and intricacy. On this chip, the concave gratings for opticalwave separation is defined using a dry etch on either crystal silicon orpolyimide. The optical elements and the photo diode array are definedusing photolithography on the same silicon substrate, eliminating thecomplicated alignment and assembling processes which are generallyrequired for fabrication of conventional spectrophotometers. In order toorthogonally intersect the light reflected from the gratings, thephoto-diode array is built on a suspended silicon bridge which is bent90 degrees from the wafer surface. In this way, the dispersed wavesignals, which on this device are designed to propagate along thesilicon wafer surface, can be very efficiently sensed by thephotodiodes. On this chip, CMOS circuitry and the photodiode array arebuilt at the same time using the same process such that output signalsfrom the photodiodes can be amplified and multiplexed on-chip,decreasing noise pick-up, and allowing conveyance of output signalsthrough a common data bus. This monolithic structure results in acompact spectrophotometer of significantly reduced size and weight. Itscost will also be lowered from the current commercial products because asimplified fabrication process is used

Additional functions, objects, advantages, and features of the presentinvention will become apparent from consideration of the followingdescription and drawings of preferred embodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 schematically illustrates in perspective view a monolithicmicrospectrophotometer having optical gratings, a photodiode array, andsignal processing circuitry integrated on a silicon chip;

FIG. 2 illustrates a releasable silicon bridge structure withphotodiodes bendable 90° from its original orientation after beingreleased from the substrate;

FIG. 3 is a representative cross section of a silicon bridgeaccommodating the photodiode array; and

FIG. 4 schematically illustrates readout circuitry for a monolithicspectrophotometer, with output signals from photodiodes amplified by lownoise operational amplifiers and multiplexed and converted into digitaldata for conveyance of outputs through a common data bus.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 is a perspective view of a micro spectrophotometer 10monolithically constructed on a silicon substrate 11 in accordance withthe present invention. The micro spectrophotometer include a concavegrating array 12, which is used for dispersing optical waves as well asfocusing reflected light onto the photodiode array 14 sited on a siliconbridge 16. The silicon bridge 16 is bent 90° from the wafer surface 20in order to orthogonally intersect the output light from the gratingarray 12. A precision notch 22 is defined the silicon substrate 11 forcoupling to an optical input fiber 24. Signal processing circuitry 26 isetched on the substrate 11 using conventional CMOS processes for initialprocessing of information received from photodiode array 14.

Advantageously, all of the optical elements, photodiodes, and relatedmicrostructures on the micro spectrophotometer 10 are defined usingphotolithography on the same silicon substrate 11. As a result, nomanual adjustment or alignment between the optical components and thedetectors is required after the IC processes for device fabrication.This is critical in reducing the manufacturing cost as well as enhancingthe precision of the system.

Conventional photolithographic and etching techniques can be used toconstruct micro spectrophotometer 10. The grating array 12 is fabricatedby a photolithographic patterning and a dry etch on either crystalsilicon or polyimide to form facets perpendicular to the wafer surface.Using advanced dry etch technologies, deep vertical microstructures canbe carved into crystal silicon in a reasonable time period. Some noveletch technologies can etch trenches as deep as 500 μm into silicon withan aspect ration as high as 100. However, the side walls of thesedry-etched trenches are generally very rough and are not suitable asfacets for optical applications. In order to improve the smoothness ofthese facets, post processes including a thermal oxidation and asubsequent wet etch for removing the oxide grown is generally requiredfor eliminating these sidewall defects. The deep silicon etch and itspost processes bring difficulty in combining this process with theintegration with other microstructures and circuitry because it isdifficult to do photolithography on a non-planar wafer surface. As aresult, processes like trench refill are generally required foralleviating the problem. Alternatively, polyimide based processes can beused, since it is comparatively easier in to shape intohigh-aspect-ratio microfacets. However, polyimide is prone to beattacked by either ethylene-diamine pyrocatechol (EDP) or TMAH etch,which is required for releasing the micro bridge on the microspectrophotometer 10. Accordingly, a reliable passivation process isrequired for protecting the polyimide microstructures if polyimidegrating is to be used on this device.

Input optical waves are coupled into the spectrophotometer through anoptical waveguide, as shown in FIG. 1. In preferred embodiments, theinput optical waves are received from optical elements directed towardcolor images such as may be created by xerographic or inkjet printers.In xerographic printing, a spectrophotometer in accordance with thepresent invention is a component of a closed-loop color control system(not shown) that enables the printer to generate reproducible colorimages in a networked environment.

In operation, optical waves received from such a printed image areguided along the surface of the silicon chip, get dispersed by thegrating array 12, and then are focused on the photodiode array 14. Inorder to efficiently couple the reflected optical waves onto thephotodiode array 14, which are originally oriented perpendicular to thewafer surface, a suspended bridge structure 16 is designed toaccommodate these diodes such that the photodiodes can be flipped 90degrees out of the wafer surface to intersect the path of the reflectedoptical signals.

One suitable two step procedure for re-orienting a photodiode array inaccordance with the present invention is schematically illustrated by aninitial and end view of a structure 40 as shown in FIG. 2. After beingreleased from substrate 41 (initial view), the orientation of a bridge46 can be rotated 90 degrees from a wafer surface 50 and be fixed atthat position by a silicon anchor 58 (end view). This structure 40 issimple to fabricate and results in better efficiency in coupling theoptical waveguide into the photodiodes than other approaches such asusing a microprism for coupling.

As illustrated with respect to a cross sectional view FIG. 3, inconsidering the fabrication process for realizing structure 40, thebridge 46 can be defined by an ion implantation and a subsequentannealing process to form a 10 μm deep p-well. In this well, another ionimplantation is used to construct p-n junctions for the photodiodes.This well/photodiode structure is made compatible with the CMOS processfor realizing the signal processing circuitry on this spectrophotometer.The choice of using n-implant in p-well but not the reverse structure asphotodiodes is based on two reasons: first is that it is easier toaccurately control the diffusion depth of n-dopants than that ofp-dopants. This junction depth control is important in achieving highquantum efficiency of converting optical signal into electrical current,especially in the shorter wavelength range, e.g., blue light whichrequires very shallow junction for efficient detection. The secondreason is because of process compatibility, e.g., the p-well structurecan be released from the substrate by boron etch stop, which is a well-established process. While a 10 μm thick p-well plate is used as thebridge for supporting photodiodes, the same thickness of silicon is toostiff to be used as a hinge which needs to be twisted for changing thebridge orientation. In order to form flexible hinges connecting thebridge to the substrate, as shown in FIGS. 2 and 3, a shallow borondiffusion (in hinge area 62) is required. Typically a 3 μm thick hinge47 will be capable of being bent easily while being mechanically strongenough to support the bridge structure.

The release of this bridge structure can be achieved by using aselective wet etchant such as EDP or TMAH, which are generally used forreleasing suspended structures in CMOS imager arrays. In order tominimize the etch time in this releasing process, the edge of the p-wellbridge needs to be aligned to the <100> direction, which has the fastestetch rate in both of these anisotropic etch. As a result, the lightlydoped silicon under the n-well can be completely undercut to form cavity60 in a short time. Typically this kind of direction dependent releasingetch can be finished in less than 40 minutes if the bridge width is nomore than 100 μm.

On a micro spectrophotometer in accordance with the present invention,the dispersed optical signal is detected by a photodiode array andconverted into electrical signal. The photodiodes are biased at a fixedreverse bias condition such that most of the carriers generated byincident light can be swept across the depletion region to provideelectrical current to the external circuit. FIG. 4 shows a circuitschematic of the readout circuitry 70 for the spectrophotometer. Asshown in FIG. 4, the output current from the photodiode is connected toa resistance on which a voltage difference appears whenever incidentlight generated photo-current through the diode. This voltage differenceis amplified by a low-noise operational amplifier, which can be designedto have a high gain and a specified bandwidth. If the detection of verylow-intensity light is required, a unit-gain buffer stage can be addedin front of the gain stage such that the signal detected is notdeteriorated by thermal noise of the input resistance R. The output ofthe opamp is then multiplexed and converted into a digital signal andconveyed out of the chip through a common data bus. With thisarrangement, the signal-to-noise ratio can be significantly increasedand as a result the capability of the chip to sense low-intensity lightat high speed is improved. With the addition of the multiplexer and A/Dconverter, more photodiodes can be put in the array for sensing thedispersed signal without an overflow in the number of the output wires.This is critical for improving the spectral range and resolution of thedevice. The signal processing circuit on this spectrophotometer will beintegrated on-chip and its fabrication process will be combined withthat of the photodiodes and the microbridge.

As those skilled in the art will appreciate, other variousmodifications, extensions, and changes to the foregoing disclosedembodiments of the present invention are contemplated to be within thescope and spirit of the invention as defined in the following claims.

What is claimed is:
 1. A monolithic spectrophotometer comprising amonolithic substrate, a grating for dispersing input optical wavesdefined in the monolithic substrate, a photodiode array movable to aposition to receive dispersed optical waves from the grating, and signalprocessing circuitry formed on the monolithic substrate and connected tothe photodiode array.
 2. The monolithic spectrophotometer of claim 1wherein the grating is defined in the monolithic substrate to extendperpendicular to a substrate surface of the monolithic substrate.
 3. Themonolithic spectrophotometer of claim 1 wherein the grating is definedas facets in crystal silicon in the monolithic substrate.
 4. Themonolithic spectrophotometer of claim 1 wherein the grating is definedas facets in a polyimide coating on the monolithic substrate.
 5. Themonolithic spectrophotometer of claim 1 wherein the grating is definedin a concave wall of a cavity defined in the monolithic substrate. 6.The monolithic spectrophotometer of claim 1 wherein the photodiode arrayis defined in a movable support structure on the monolithic substrate toextend perpendicular to a substrate surface of the monolithic substrate.7. The monolithic spectrophotometer of claim 1 further comprising ananchor defined in the monolithic substrate, and wherein the photodiodearray is defined in a movable support structure permanently lockable ina fixed position using the anchor.
 8. The monolithic spectrophotometerof claim 1 further comprising a bridge defined in monolithic substrateto extend across a cavity in the monolithic substrate, and wherein thephotodiode array is defined on the bridge.
 9. The monolithicspectrophotometer of claim 1 further comprising a bridge defined inmonolithic substrate to extend across a cavity in the monolithicsubstrate, and wherein the photodiode array is defined on the bridge toextend perpendicular to a substrate surface of the monolithic substrate.10. The monolithic spectrophotometer of claim 1 further comprising anotch defined in monolithic substrate to hold an optical fiber for inputoptical waves.
 11. A monolithic spectrophotometer comprising amonolithic substrate, a grating for dispersing input optical wavesdefined in the monolithic substrate, a suspended bridge defined over acavity in the monolithic substrate, the suspended bridge movable to aposition to receive dispersed optical waves from the grating, and aphotodiode array defined on the suspended bridge to receive dispersedoptical waves from the grating.
 12. The monolithic spectrophotometer ofclaim 11 wherein the grating is defined in the monolithic substrate toextend perpendicular to a substrate surface of the monolithic substrate.13. The monolithic spectrophotometer of claim 11 wherein the grating isdefined as facets in crystal silicon in the monolithic substrate. 14.The monolithic spectrophotometer of claim 11 wherein the grating isdefined as facets in a polyimide coating on the monolithic substrate.15. The monolithic spectrophotometer of claim 11 wherein the grating isdefined in a concave wall of a cavity defined in the monolithicsubstrate.
 16. The monolithic spectrophotometer of claim 11 wherein thesuspended bridge is movable to extend substantially perpendicular to asubstrate surface of the monolithic substrate.
 17. The monolithicspectrophotometer of claim 11 further comprising an anchor defined inthe monolithic substrate, and wherein the suspended bridge ispermanently lockable in a fixed position using the anchor.
 18. Themonolithic spectrophotometer of claim 11 wherein the suspended bridge isconstructed from p-doped silicon.
 19. The monolithic spectrophotometerof claim 11 further comprising signal processing circuitry formed on themonolithic substrate and connected to the photodiode array.
 20. Themonolithic spectrophotometer of claim 11 further comprising a notchdefined in monolithic substrate to hold an optical fiber for inputoptical waves.